Field of the Invention
The invention relates to a surface acoustic wave (SAW) filter of the resonator type, which is suitable as a bandpass filter with low losses and a high cut-off band attenuation ratio, in particular as an input or output filter for cordless or mobile telephones.
Such an SAW filter is known, for example, from German Patent DE 44 08 989 C2. That filter has a large number of surface acoustic wave resonators, which are connected in series and parallel with one another. Each of the resonators includes an interdigital transducer, which is disposed between two reflectors. Both electrode fingers of the interdigital transducers and reflectors (which are disposed, for example, like a grid) have a finger periodicity which corresponds essentially to half a wavelength of a resonant frequency.
Consequently, not only the surface acoustic waves produced at the individual electrode fingers, but also the surface acoustic waves reflected at the individual reflectors can be optimally amplified by constructive interference. An RF signal can thus be transmitted virtually without any losses from one connection of the surface acoustic wave resonator to the other. The series and parallel connection of a plurality of resonators also results in frequencies outside the pass band being further attenuated, and a cut-off band suppression thus being increased. Outside the pass band, SAW filters of the resonator type provide optimum cut-off band suppression, if the finger periodicity of the reflectors and of the interdigital transducers is in each case the same for all of the interconnected series resonators and for all of the interconnected parallel resonators. The above-mentioned German patent also proposes that the finger periodicity, or the distance between the electrodes in the reflectors, be set in such a way that it is not the same as the distance between the electrodes in the interdigital transducers. That proposal relates, in particular, to the distance between the electrodes in the reflectors of those resonators which are connected in series with one another. That results in the elimination of the interference spike, which occurs outside the pass band in known filters.
However, in those and other known SAW filters of the resonator type, it is regarded as a disadvantage that the edges of the pass band on the pass band curve do not fall sufficiently steeply, and that the transition between the pass band and the cut-off band is not sharp enough, so that the pass band curve falls too slowly in that region, or has excessively rounded edges.
It is accordingly an object of the invention to provide a surface acoustic wave (SAW) filter of the resonator type having an improved edge steepness, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, which has good cut-off band suppression and which nevertheless has a sufficiently broad pass band with steeply falling edges.
With the foregoing and other objects in view there is provided, in accordance with the invention, a surface acoustic wave (SAW) filter with two ports, comprising a substrate having mutually parallel tracks for surface acoustic waves; at least three resonators in the tracks, each of the resonators having reflectors, an interdigital transducer disposed between two of the reflectors, a pair of electrical connections, and a plurality of electrode fingers disposed at a finger periodicity; the resonators including resonators connected in series through the connections and resonators connected in parallel with the resonators connected in series; and the resonators including a plurality of resonators of at least one resonator type, at least two of the plurality of resonators differing with respect to the finger periodicities of the interdigital transducers.
The SAW filter according to the invention has at least two resonators, which are connected in parallel and/or in series with one another, and in which either the series-connected resonators and/or the parallel-connected resonators have at least two different finger periodicities of the interdigital transducers. The magnitude of this discrepancy in the finger periodicities allows a greater pass band edge steepness to be achieved deliberately and, in particular, a sharper transition to be achieved from the pass band to the cut-off band. Furthermore, a flatter pass band is provided, with the forward loss being low and unchanged. This result is surprising and contradicts the values which one has been led to expect for such a configuration. A shallower edge steepness is actually expected for resonators with a different finger periodicity than for filters with resonators connected in parallel and series and which have the same finger periodicity. However, the invention provides an SAW filter with an edge steepness that at least remains the same.
It is furthermore possible to deliberately make one of the two edges of the pass band steeper with an SAW filter according to the invention. This may be that edge which bounds the pass band toward higher frequencies, or else that edge which bounds the pass band toward lower frequencies. On one hand, this is achieved if different finger periodicities are set only for series-connected resonators, with the left-hand edge, which points toward lower frequencies, being set to be steeper. On the other hand, by differently setting the finger periodicities in the parallel-connected resonators, the right-hand edge of the pass band, which is the edge that points toward higher frequencies, is made steeper. Furthermore, the transition from the pass band to the edge (for example in the relative attenuation range from 0 to 3 dB) is more abrupt.
In accordance with another feature of the invention, at least three different finger periodicities are selected for series-connected resonators Rs1 to Rsn (where nxe2x89xa73), in which case the resonator having the greatest finger periodicity is disposed on the outside in the series circuit, while the resonator having the smallest finger periodicity is disposed in the center. If there are more than three series-disposed resonators, the resonator having the smallest finger periodicity is preferably not disposed adjacent the resonator having the greatest finger periodicity.
In accordance with a further feature of the invention, there are provided four series-connected resonators, in which case each series resonator forms a basic member with a resonator connected in parallel with it, the parallel-connected resonators in the basic member can be assigned either to only one or to two adjacent basic members, three different values are set for the finger periodicity of the electrode fingers of the series resonators, an outer resonator in turn has the greatest finger period, the resonator having the smallest finger period is located on the inside, and the other outer resonator and one inner resonator have the same finger periods. Finger periods P1 to P4 for four series-disposed resonators R1 to R4 are thus: P2 less than P1=P3 less than P4 or P3 less than P1=P2 less than P4, or in the opposite order in each case.
In general, it can be said that better cut-off band suppression is achieved with a greater number of series-connected resonators. In the case of SAW filters according to the invention, and with a given number of parallel-connected resonators, the maximum number of series-connected resonators are selected, in order to obtain a maximum variation capability for the configuration of the finger periodicities, with the minimum total number of resonators. For example, an SAW filter according to the invention having three basic members can be formed by two parallel resonators and three series resonators, and an SAW filter according to the invention having four basic members can be formed with four series resonators and two parallel resonators.
Analogously, a filter according to the invention may include three basic members with a maximum number of parallel resonators and a minimum number of series resonators. In that case the finger periods of the interdigital transducers in the parallel resonators have at least two different values, but three different values for three or more basic members.
Furthermore, additional series-connected resonators may be provided, which have no adjacent parallel-connected resonator, and/or can form one basic member with no resonator connected in parallel with it.
In accordance with an added feature of the invention, at least one series resonator is replaced by two parallel-connected resonators, having respective inputs and outputs which are connected to one another.
In accordance with a concomitant feature of the invention, with regard to the differences to be set for the finger periodicities, discrepancies of 0.1 to 3 percent have been found to be suitable, and 0.5 to 1 percent have been found to be particularly advantageous. While the characteristics and, in particular, the pass band of the filter become poorer again as the discrepancies in the finger periodicity become greater, the effect according to the invention, namely the steeper setting of the edge pass band, is reduced as the discrepancies in the finger periodicity become smaller.
The specific setting of a higher edge steepness for only one edge is advantageous if two adjacent frequency bands have to be separated from one another. For example, the mobile radio network for the GSM Standard uses different frequency bands for transmitters and receivers. Those are separated from one another by, for example, 20 MHz. If SAW filters according to the invention are now used as input and output filters, then it is possible, specifically, to construct those edges of the pass bands to be steeper which bound the pass band to the adjacent pass band for the transmitter or receiver band. Since the transmitter band in the GSM Standard is lower than the receiver band, filters with a steep right-hand edge (toward higher frequencies) are advantageously used for the transmission band, and filters with a steep left-hand edge (toward lower frequencies) of the pass band are used for the receiver band.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a surface acoustic wave filter having an improved edge steepness, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.